High-frequency broadcasting over power lines



R'. D. DUNCAN, JR HIGH'FREQUENCY lBROILDCASTING OVR POWER LINES 0st. s, 1929.

rents a practically inoperative broadcast system would result.v as may beseen from the foiloiving considerations. First assume that each of the independent high frequency oscillators are to he operated at thel saine frequency. lt is well-known in the radio nrt that itis impossible to maintain even. two inde- .l pendent high frequency oscillators in exact frequency synchronizationand that there is an ever present tendencv for one oi' the other to shift 1n frequency. ne practical result of j this is that relatively small differences in frequency which would be bound to exist het weer. the several oscillators would result iii disturbing beat tones between the various tandis mental and harmonic -frequencies being vheard at some or all ofthe coiisunicrs receivers dus to the inductive relationwliicli frequentlyY exists between lines'extending from different substations Where the areas served by these diierent substations either overlap or the transmission lines serving them are ni close provided, in shunt tothe transformer or transformers at the substation. These transformera provide eilicient high frequency paths which function as feedback couplings 'between the output and input sides of the amplifier.V l

The improved arrange-ment avoids the attenuation whichY might otherwise resultfrom tion with respect to all of the several substat-ions so lthat. the distances from the broadcasting station to the various substations may geographical proximity. Manifestly Where there are a large number of snbstations, say teen ortwenty, las-is` the case ivithplarge" not be greatly out of proportion one to theother. Of course, where lthere are a' large cities, the in terfereiice arising from 'frcquenc y non-synchronization would be prohibitive. Second7V assume that the fundamental frcquent-ies of .the high frequency oscillators located at the separate snbstations .vvere'dis placed by frequency increments so that the resulting 'nent tones would be above audibil- 1ty. vThe practical result of this procedure t* i would be `vvliere there are a plurality of sub stations' each served by aseparate oscillator, that a -fast range of frequencies wvoald'lie required. It is fundamental in the Wired radio aat that only a rather restricted hand of the lovver range of carrier frequencies may be cectively utilized for transmission-even tho y this were not the case, to design, construct and maintain vast numbers ofreceivers, each responsive to va wide range of frequencies,

fsuch -as this arrangement would require,

sons previously set forth.

would oe-manifestly unecononiical and out of the question. Furthermore, even tho the 'va-V rious fundamental frequencies were displaced by super-audible increments, the ever present harmonic frequencies which it is substantially impossible to suppress, .vould themselves give rise to a host of disturbing beat tones in the many-consumers receiving sets for the 'rea- In systems according to my previously mentioned copending` application, wherein the high frequency transmitter is located at the central power station, it has been found to be a very diicult matter to provide ampliication at thesu'ostationon account of the 'tendency' for such amplifiers to operate regeneratively,' thereby generating* oscillations which heat with the. carrier :trepiencyi4 nuniberrjof.substations, there is likely to 'be a considerable ditlerence'in the distances from the broadcasting station to the nearest and farthest substation. This difference, however, is offset, in accordance with the present invention, by providing various degrees of amplifica-tion at the different substations, depending upon their relative distancesfrom thebroadcasting centralstation and, as before stated, npon'tlie number of consumers and area servedhy the individual substations. It is 'thought that the invention will he more clearly understood from the following detailed'de'scription 'With reference .to the -accoinianyinp; drawings, in which A. Fi'grl is a diagrammatic illustration of van alternating;r current power distribution systcm comprising a central power station and two snbstations with a number of distrih- 'uting circuits Veirtendiiigfrom each of the substations to various consumers installations together with a broadcasting Acc-initial station, and 'Y f Fig. 2 is a circuit diagram illustrating fa high frequency ampl-ier inserted in a high frequency carrier current transmission line extending), from a. broadcasting central sta` vtion to one of the 'substations Referring to Fig. l, a central power station l, together with two substations 2 and 3, are shown interconnected. An alternar ing current generator 4 is indicated at the central posici-'station l. The output of this generator is transmitted through the medium of the. regulating and rrietering devices 6 to the bus hars 7. 8 and 9, from which tivo three wire.I three-pliase transmission circuits 'l0 andA il extend to the substat-ions '2 and 3 lrespectively. Atthe substationsare shown three phasetransforiners l2 and 13 respectivelylto which the primary windings of the transmisgli i waait-ie sion circuits 10 and 1l are connected. The terminals of the secondary Windingsof transformer 12 are shown connected through the medium of metering and regulating appa-- ratus 6 to conductors 14, 15 and 16 and thence to bus bars 17, 18 and 19, respectively. Three single-phase circuits 20, 21 and 22 are shown connected to the bus bars at the substation.

These single-phase lines constitute branch 'at a considerable distance from the broaddistri-buting circuits for alternating power V and lighting current. In accordance with the usual practice, each consumers installationis connected With a branch circuit through the medium of'a step-down transformer such as transformers 23, 24, 25 and 26. 'Substation 3 is suhsta'nt'iallv identicalyvith ,substation 2'. Here the terminals of the secondary Winding of transformer 13 are con- .A nected throuvh metering and' regulating'apparatus 62 aia'id conductors 27, 28 and 29 to bus bars 30. 31 and 32, respectively. From these latter bus bars there are shown extending three single-phase branch circuits similar to the branch circuits 20, 21 and 22 extending from substation 2.

A high frequency broadcasting central station is indicated at 33. This station' com prises. a high frequency carrier current generator 34, a microphone 35, and a modulator 36. lThe high frequency generator may be of any suitable type such as a vacuum tube oscillator or a high frequency rotary encrator, and modulation may be edected. y any one of the several Well-known methods. The modulated high. frequency carrier current is transmitted from the broadcasting central station- 33 over the lhigh frequency transmission'lincs 3T and 3S to the-substations 2 and 3.' it substation the two conductors of the high frequency transmission line 37` are shown connected respectively to conductors 15 and 16' through the medium of condensers 39 and 40.. These latter condensers function to prevent short circuit of the louY frequency alternating currentwfroml the conductors 15' and 1&5 to the apparatus at the broadcasting central station, and may be utilized for tuning. the line to the carrier current frequency.

Asstated in my previously referred to copending application, I have discovered that ig'h frequency signaling current may be transmitted over three-wire, three-phase power transmission lines and received from any two of the three wires although the high frequency apparatus is connecte-d direct-ly only 4to two of the three Wires comprising the three-phase system. With connections as shown in Fig. l, the high frequency carrier current finds its Way to all the associatedA branchdistribution circuits 20, 21 and 22, over which it isl transmitted tothe Yarious pours-r consumers installations suchY as indicated at 41, 42, 43, etc. At the consumers station #lil there is indicatori apparatus tt adapted to receive and detect modulated high frequency carrier currents. This apparatus includes aloud speaking telephone instrument. The receiving terminals of the apparatus 44 are connected, preferably, through condensers 45 and 46 to the power lines 47 and 48 respectively., The other consumers installations such: as 42 and i3 are similar, in general, to that of 41.'

ber of consumers than in the case of other f substation areas. When these conditions exist, itis desirable ,to increase the amount of high frequency energy available'as compared with the energy required to serve a considerably smaller number of consumers.- To moet the requirements in each instance, the number of steps of amplification may be increased or decreased as the case may be. Generally it will be necessar to provide at least one stage of amplification, but where the attenuation is low and the number of consumers is relatively small, the amplifier may he dispemsed with'. v

Fig. 2 is a.l reproduction-of a portion of Fig,

'1 on a much larger scale including details of a suitable amplilicr circuit. There is shown a conventional single-stage radio frequency amplifier or repeater comprising two threeelectrode vacuum tubes 50 and .51 with associated circuit connections familiar to those skilled in the art. Condensers 52 and 53 correspond to condensers 39 and 40 and have similar functions, namely, to prevent the short circuit of the low frequency alternating current from the power transmission lines and to tune the power transmission lines tothe frequency of the carrier current.

The various branch distribution circuits and consumers installations associated yWithsubstation 3 are similar, in general, to those shown associated with substation 2, and re.

quire nov discussion. Y

In some cases it may be found that the amplifiers operate as oscillators due to feed-l bark connections via the power transmission lines. For example, in Fig. 1 the output and input! stations of the' amplifier 49 are coupled through transformers 12 and 13, conductors l0 and l1, and bus bars T, 8 and 9. It the attenuation of this coupling path is sui-` ciently low, oscillations lWill occur. To

. overcome the oscillatory condition, radio frequency choke coils 541 may be inserted any um n fiil-v current.

" Although only a three-phase system isl shown, the utility of the invention is not limited in 'any sense to three-phase systems; but is equally applicabh` to single phase and multi-phase systems ot' all degrees.

I claim:

1. A high frequency broadcasting system comprising an electric power distribution network including a central power generating station, a plurality of substations connected to said central station, and branch distribution circuits extending from said substations to power consumers installations, a high frequencyv broadcasting central station, Vva plurality of high frequency carrier current transmission lines extending from said broad-` casting lstation to ditl'erent substations, said transmissionA lines being'conne'cted at said substations to said branch distribution circuitsV forV impressing modulated high irequency current upon said branch distribution circuits, means for preventing.,Ir undesired oscillations between said central station and said substations and high frequency signal receiving devices at a plurality of said con? sui-ners installations. y

2.1m a higlidfrequency broadcasting sys-l I A V'signals transmitted from Said broadcasting tem, an electric power distribution network comprising a plurality of distribution areas, a power generating central station, a-plurality of substations, power transmission lines connecting said central station with said sub-V stations, each of said substations serving one of said areas, a plurality or" branch dist-ribution circuits extending from each of saidv substations, and a plurality of power consurn ers7 installationsconnected to the said branch circuits of each area, a higlrfrequency broadcasting central station comprising means for e generating and modulating high frequency carrier current in accordance with signals to be broadcast7 a plurality of carrier current' transmission lines extending from said broadcasting station, each of said carrier current transmission lines extending to a different one of said distribution areas and operatively associated with the branch distribution circuits o'tsaid area for impressing modulated high frequency current upon said branch circuits for eil'ecting the transmission of high frequency signals theieovcr, means for preventing the circulation ot' undesired oscillations between said central station and said substations, and high frequency signal receivingidevices at a plurality of said consumersA installations, said devicesbeing connected to Sarti branch distribution circuits and adapted to receive signals transmitted from said broadcasting central station.

" 3. In a high frequency broadcasting sfs-` teni, an electricpower distribution network of substations connected to said central generating station,- each of said substations being individual to one of said distribution areas, a plurality ol branch distrilu'ition circuits extending from each of said substations, a plurality of power consumers"V installations connected to said branch distribution circuits,

` a high frequency broadcasting central station comprising means for generating and'inodw lating high frequency carrier current in accordance with 'signals to be broadcastaV plurality ot' carrier current transiuissioulines each extending from said broadcasting central station to a dilt'erent one of said substa- Vtions -and operatively associated with the branch distribution circuits extending therefrom for impressing modulated high frequency signaling current upon said branch distribution circuits wherebyl high irequency signals can be transmitted troni said broad casting central stations to said consumcis.

ldevices being'connected to said'- branch distribution circuits and adapted to receive central station, means disposed in the circuits lui between said central station and said sulosta-A 1 tions for preventing the. circulation of undesired oscillations in said circuits, and means for amplifying highfrequciicy carrier cur- Sui:

rents interposed in at least one of said carrier current transmission lines.

el. ln a high frequency broadcasting system, an electric power distribution network comprising a plurality of distribution areas, a

central power generating station, a plurality of substations connected to said central gcnerating station, each ot said substations being individual to one/of said distribution areas, a plurality ot branch distribution cir cuits extending from each of said substations,a plurality ot' power consumers` installations connected to said branch distribution circuits, a high frequency broadcasting central station comprising means for generat-4 ing and modulating hio'h frequency carrier currentin' accordance with signals to be broadcast, a plurality of carrier current transmission lines each extending `from said roadcastiug central station to al dl'l'erent one of said substations and operatively associated with the branch. distribution circuits extending therefrom for impressing niodli` 1,730,412 fw r5 nected in series with said carrier. current transmission lines for preventing undesired oscillations in said lines whereby high frev quency signals can be transmitted from said broadcasting central station to said consumers installations through the medium of said carrier current transmission lines and said branch distribution circuits, high frequency signal receiving devices at a plurality of said consumers installations, said devices being connected to' said branch distribution circuits and adapted to receive signals transmitted from said broadcasting central station, and means for amplifying' high frequency carrier currents interposed in at least one of said carrier current transmission' lines, in at least one of said substations, y

5. In a high frequency broadcasting sys- I tem, a multi-phase power generating central station, a plurality of multi-phase power transmission lines extending from said centrai station, a plurality of substations, each i of said substations including a multi-phase transformer, the primary windings of each of said transformers being connected to one of said power transmission lines, a plurality of branch distribution circuits associated Y with each of said substations, said branch distribution circuits extending to consumao'ei's installations, a broadcasting central station comprising means for generating and modulating high frequency carrier current, aplurality of high frequency carrier current transmission lines extending from said broadcasting central station, each to one of said substatioiis,` said high frequency transmission lines being connected tothe output circuitsof the transformers at their respectively associated substations for impressing modulated high frequency signaling current 'on said branch distribution circuits, means connected in series with said lines for preventing the circulation of undesired oscillations therein, means' at certain of said substatiors for amplifying the high frequency carrier current means operable to prevent the flow of low frequency current from the power transmission system to the high frequency carrier curi. rent transmission lines, said means being inv operative to prevent the flow of high frel quency carrier current from said high frequency carrier current transmission lines to the power distribution system, and means at a plurality of consumers installations for receiving and indicating modulated high frequency/"carrier current transmitted over the power distribution system. y G. ln a high frequency broadcasting system, a central powerge'nerating stat-ion, ,a`

plurality of substations, conductors for the transmission of power current interconnectsaid eentralgenerating station with said substations, branch distribution circuits extending from each of said substations to a 55 vplurality of consumers installations, ahigli frequency broadcasting central -Staticmbiglil frequency carrier current transmission lines `extending from said broadcasting central station, each to one of said substations, .a high frequency amplifier' interposed in .one of said high frequency carrier current transmission lines, thc arrangement being such that there is an inherent feedback path from the output side of said amplifier to the input side' thereof h way of the conductorsintercon; necting t e central power generating station and the substations andimpedances intioning as an oscillator by `reason of said feedback path but inoperative to substantially impede the How of low frequency power v current. 7. In a high frequency. broadcasting system, a central power generating station, a plurality of substations, conductors for the transmission of power current interconnecting said central generating station with said substations,lbranch distribution circuits eX- tending froin each of said substations to a scitcd in said conductors, said impedances being effective to prevent said amplifier funeplurality of consumers installations, abigh Y requeiicy broadcasting central station, l iigh frequency carrier current transmission lines extending from said broadcasting central t station, each 'to one of said substations, a high frequency amplifier interposed in'one of said high frequency carrier current trans-A mission lines, the arrangement being such that there is an inherent feedback path fromV the output side of said amplifier to the iiiput side thereofby way 0 interconnecting the central power generating station and "the substations and choke the conductors f coils inserted inv said conductors, said choke coils beingY effective to prevent 'said amplifier functioning as an oscillator'by reason of said feedback path but inoperative to substantially impede the flow of low frequency l power current.

S. In a wired radio broadcasting system the combination of an electric power distribution net work comprising a plurality of distribution areas, a central power generating station, a plurality of power substations connected to said central station, each of said substations being individual. to one of said distribution areas, a plurality of branch disiis ti'ibution circuits extending from each of said substations, a plurality of power constations, connections between said carrier current transmission lines and the input circuits of said power amplifier, means in said connections for suppressing undesired circulatory oscillations, additional Connections between said branch distribution 'circuits and the output circuits of said power amplifiers, said first mentioned and additional connections providing means for impressing amplified modulated high frequency signaling current on said branch circuits whereby high frequency signals may be transmitted from said broadcasing central station to said consumers installations thru the medium of said carrier current transmission lines, said power amplifiers and said branch distribution circuits, high frequency signal receiving and reproducing devices at a plurality of said consumers installations, said devices being operatively connected with'branch distribution circuits.

9. In a wiredradio broadcasting system the combination of a polypha-se electric power generating central station, a plurality of polyphase transmission lines extending from said central station, a plurality of substations, each vot said substations including at least one polyphase transformer, the input winding of said transformer being'connected to one'of said polyphase transmission lines, a plurality of branch distribution circuits associated with each of said substations, connections between 4said branch distribution circuits and the output windings of said pol yphase transformer, a plurality of power consumers installations, aplurality of additional transformers, connected to said branch. distribution circuits, additional connections between said consumers installations and said additional transformers, a broadcasting cen tral station comprising mea .s for generating and modulating high frequency carrier current, high frequency carrier current transmission lines extending from said broadcasting central station to said substations, high frequency power amplifiers including input and output windings, located in at least one ofsaid substations, connections between said carrier current transmission lines and the input windings of said power amplifier, means in said connections for suppressing undesired oscillations, additional connections between said branch distribution circuits and the output windings of said power amplifier for ini pressing amplified modulated'high frequency signaling current on said branchY distribution circuits, high frequency signal receiving and reproducing devices at a plurality of said consumers installations, said devices being operatively connected with said additional transformers whereby highV frequency sig nais transmitted from said broadcastii'ig central stations are rendered audible at said sig- 'nel receiving and reproducing devices at said consumers installations.

l0. In a wired radio broadcasting systeu `the combination ot a polyphase electric powc generating central station, a plurality oi polyphase transmission lines extending trou said central station, a plurality of substations cach of said sulistations including a plural ity of transformers connected in a polyphasa l transformers, a broadcasting central station comprising means for generating and modulating high frequency carrier current, high -frequency carrier current transmission lines extending from said broadcasting central station to said substations, high frequency power amplifiers including input and output windings located in at least one of said sub stations, conn ections'betweeu said Carrier cur` rent transmission lines and the input windings of saul power amplifier, means 1n said connections for suppressing undesired 0scil lations, additional connections between saidbranch distribution circuits and the output windings of said power amplifier for impressY lng'amplified modulated high frequency s1g-" nalinglf current on said' branch distribution circuits, high Vfrequency signal receiving and reproducing devices at a plurality 'of said consumers installations, said devices being operatively connected with said additional transformers whereby high frequency sig nals transmitted from said broadcasting central stations are rendered audible at said signal receiving and reproducing devices at said consumers? installations.

-Y 11. A system for broadcasting signaling energy over electric power transmission systems comprising means for generatinghigh frequency carrier current. means for modulating the carrier current in accordance with audible signals, means for transmitting the modulated carrier current to a pluralitlyT oi power substations, a central station, trans mission wires interconnecting said substations and said central station, means for rais ing the level of the high frequency modulated carrier current at one or more of the substations, means for preventing the undesired circulation of high frequency energy through the path formed by the interconnection of said central station with said substations over Y said transmission wires and means for iinpressing the modulated carrier current at increased amplitude on out going power I ing between said central station and said sub i transmission lines at selected power substations.

l2. A system of wired radio signal trausmission over electric power transmission systems comprising means for generating high frequency carrier current, means for transmitting the generated carrier current to a i i vided by sa plurality'of power substations, a central pow- 'er station, transmission wires interconnecting saidcentral power station with said substations, means for ampllfgmg the carrier current at one or more su statlons', meansfor y preventing the undesired circulation of 'energy .between the inputand output circuits of said amplif ing means over the path protransmission wires betweenV said central station and said substations and means for impressing the amplified carrier current on the outgoing power transmission lines at selected power substations. v A 13; Awired radiobroadcasting system comprising in combination a central polyphase electric power'generating station, a plurali ity of substations located in a restricted area in geographically'spaced positions with'rei spect to said central station, a polyphase electric power-distribution network extending betweennsaid central station and said substations, a plurality of branch circuits extending from said substations to a plurality fsubscriber stations, transforming equipment at each of said substations, a high frequency central controlstation having line circuitsextending to each of said substations and connected with each of said' branch l circuits, means'in said line circuits for sup.-

'pressing undesired4 circulatory oscillations, a generator of high frequencyoscillationsat said control station, means for modulating current over said line circuits to eachvof said said oscillations for Supplying high frequency branch circuits for transmission to said subsaid substations for raising the level of the high frequency current impressed upon vthe corresponding branch circuit in proportion to thenumber of subscriber stations supplied from said branch circuits. v

14. A wired radio broadcasting system comprising in combination a central polyphase electric lpower generating stati-on; a plurality of substations located in a restricted areain geographically spaced positions with respect to said central station, a polyphase electric power distribution network extend stations, a plurality of branch circuits extending from said substations to a plurality of subscriber stations transforming equipment at eaclrof said substations, a high frequency central control station having line circuits extending to each of said substations and connected with each of said branch circuits, means. in said line circuits for suppressing undesired circulatory oscillations, a. generator ofl high' frequency oscillations at, said l .scriber stations and means at selected ones of control station, means formodulating said oscillations for supplying high frequency current over said line circuits to each oi said branch circuits for transmission to said subscriber stations, and a high frequency electron tubev amplification system located at selected ones of said substations Yand interposed between said line circuits'and said branch circuits for` controlling the. level of the high frequency current impressed upon the corresponding branch circuit in proportion tothe number of` subscriber stations sup-` plied by said branch circuits. A

- In testimon whereof I aflix my signaturev cenar nnuucann, y 7 ,c 

